Work will continue on the requirements for binding of primer tRNAs (such as tRNA Trp) to the reverse transcriptase of RNA tumor viruses (AMV). This includes investigation of the effects of temperature and salt on the stability of binding, and chemical modification of the specific nucleotides in the tRNA. We shall also continue to study the interaction between the primer tRNAs and the template RNAs. We plan to determine the nucleotide sequences at, and on either side of, the primer binding sites of MLV-RNA and RSV-RNA.